Archean

The Archean (IPA: /ɑːrˈkiːən/ ar-KEE-ən, also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history. It lasted from 4,031 to 2,500 Ma (million years ago). This ancient time period came after the Hadean Eon and was followed by the Proterozoic and the Phanerozoic. During the Archean, Earth was very different from today.

Back then, Earth was mostly a water world covered by deep oceans. There were bits of continental crust, but most of it lay beneath the water. The oldest pieces of land we see today come from this time. The air was also very different, filled with gases like methane and without any free oxygen.

Life first appeared during the Archean. Simple, tiny microbial mats called stromatolites lived in shallow water. These were made by very small prokaryotes like archaea and bacteria. Later, some of these tiny organisms learned to use sunlight through photosynthetic processes, especially early cyanobacteria. This big change started to alter the oceans and the air forever after the Archean ended.

Etymology and changes in classification

The word Archean comes from the Greek word arkhē, meaning 'beginning, origin'. Earlier scientists thought the time before the Archean had no life, but fossils were later found there.

The start and end of the Archean Eon are set by exact dates rather than rock layers. The International Commission on Stratigraphy says the Archean began about 4,031 million years ago, marking the oldest known intact rocks on Earth.

Geology

When the Archean Eon began, Earth was much hotter, partly because of leftover heat from when the planet formed and partly because of radioactive elements breaking down. This extra heat made Earth's mantle warmer than it is today.

The oldest rocks we can find on Earth's surface are from the Archean. These rocks are found in places like Greenland, Siberia, the Canadian Shield, and Australia. They include many types of rocks, such as granite and volcanic rocks. There was also a lot more volcanic activity back then than there is now. Plate tectonics—the movement of Earth's outer layers—was happening, but it worked differently because Earth's mantle was hotter. Scientists think that early in the Archean, Earth might have had small continents, but most of it was covered by deep oceans.

Environment

The Archean atmosphere had almost no free oxygen, with levels far lower than today. There were brief periods when oxygen levels rose, possibly marking early stages of a major change called the Great Oxygenation Event. The ocean during this time lacked layers with different oxygen levels and was rich in certain heavier oxygen isotopes.

Scientists believe the Sun was dimmer back then, but Earth still had moderate temperatures. This might be because of more greenhouse gases in the atmosphere or because Earth reflected less sunlight. Evidence of liquid water exists from ancient, changed rocks.

Early life

Main article: Earliest known life forms

For details on how life got started, see Abiogenesis.

The Archean Eon was a time when life first appeared on Earth. Scientists think life may have started either at the end of the Hadean Eon or early in the Archean Eon. The earliest signs of life are tiny pieces of graphite found in very old rocks in Western Greenland. These pieces show that very small living things were around 3.7 billion years ago.

The oldest fossils we know of are called stromatolites. These were built by tiny blue-green bacteria called cyanobacteria in shallow water. The first stromatolites are found in rocks that are 3.48 billion years old in Western Australia. These bacteria were important because they helped fill the air with oxygen, which made Earth more friendly for later life.